Edematous conditions, i.e., excessive accumulation of fluid in tissues, are painful conditions that can arise from a variety of causes. For example, preoperative, operative and postoperative immobilization of limbs can cause blood stasis and venous thromboembolism, a serious edematous condition. The swelling of limbs in edematous conditions can be unsightly and ultimately life threatening.
It is well known to treat edema with pressure devices that squeeze the limb, typically by means of an inflatable pressure cuff wrapped around the limb. The pressure device moves excess fluid from engorged tissues from distal portions of the limb to proximal portions, eventually to the trunk of the body where the fluids are absorbed in the circulatory system and excreted from the body. These pressure devices thus perform external, non-invasive compression therapy.
The prior art uses basically four different techniques to apply the pressure, known as wave forms. The first type of wave form is non-sequential and non-gradient and is shown in FIG. 1A. A pressure cuff is wrapped around the limb and inflated to a certain pressure for squeezing the limb and forcing the excess fluid to flow proximally (at least supposedly) to the trunk of the body. This wave form is of course quite simple and basic, but suffers from several drawbacks. First of all, there is quite a bit of discomfort at high pressures. Secondly, the pressure cuff collapses all veins and lymphatics throughout the limb, which can have deleterious side effects. Thirdly, as the length of the pressure cuff increases, the more distal regions of the limb are not effectively decongested of fluid. Fourthly, the somewhat uniform application of pressure does not effectively direct the flow in the proximal direction.
The other three wave forms recognize that in edematous conditions the fluid flow is poorly directed in the proximal direction, and each wave form attempts to solve the problem in a different manner. All three remaining wave forms employ a pressure cuff constructed with a plurality of inflatable pouches, called cells, located along the length of the limb, there being a most distal cell and a most proximal cell. What distinguishes the three other wave forms of the prior art from each other is the manner in which the cells are inflated.
The second type of wave form is sequential and non-gradient and is shown in FIG. 1B. The most distal cell is inflated, followed sequentially by the second most distal cell and so on up to the most proximal cell. The most distal cell is maintained inflated all the time up to and including inflation of the most proximal cell in order to reduce backflow, i.e., distally directed flow. However, this method still has the disadvantage of discomfort at high pressures. The most distal cell is particularly painful because it is inflated during the entire therapy. In contrast, the most proximal cell is not inflated long enough. Moreover, as soon as one cell inflates, it acts as a barrier to the more distal cells and hinders the fluid from draining proximally.
The third type of wave form is sequential and gradient and is shown in FIG. 1C. The most distal cell is inflated to a relatively high pressure, followed sequentially by inflating the second most distal cell to a slightly lower pressure and so on down to the most proximal cell. The distal cells remain inflated throughout the therapy cycle to reduce backflow. However, this wave form still has the disadvantage of the most distal cell being inflated too long and the most proximal cell not being inflated long enough. Moreover, in order to ensure creating a pressure gradient from the distal end to the proximal end, the most distal cell must be inflated to a very high pressure and the most proximal cell is usually insufficiently pressurized, thus impairing the efficiency of the method.
The sequential and gradient wave form is used in several US Patents. U.S. Pat. No. 4,370,975 to Wright describes apparatus for promoting flow of a body fluid in a human limb. Timing valves are used to inflate the cells in accordance with a timing control sequence.
U.S. Pat. No. 5,575,762 to Peeler et al. describes a gradient sequential compression system and method for reducing the occurrence of deep vein thrombosis. The system has a controller that includes a plurality of feeder valves pneumatically connected to each of the cells and a microprocessor-based control unit for opening only one of the feeder valves at a time during an inflation cycle, so that each of the cells can be independently inflated to predetermined pressure levels. The controller regulates the pressures in each of the cells by repeatedly measuring the pressures and making any necessary adjustments.
U.S. Pat. No. 5,591,200 to Cone et al. describes apparatus for treating edema by applying pressure to a patient's limb. The apparatus includes electrically-operated valves for controlling inflation of the cells. A computer individually controls each valve to variably pressurize the cell in a variable sequence. The computer also has a determiner for determining the girth of the limb being treated.
In all of the three abovementioned patents, although pressure measuring means are provided, the pressure remains substantially constant for each time interval. The inflation of each cell is controlled only as a function of time, not pressure. Wright uses timing valves. In Cone et al., it is essential to use a timer for measuring the time period for filling each cell and for generating a timing signal in response to the cell filling. In Peeler et al., as mentioned above, the pressures in each of the cells are repeatedly measured. However, the pressure measurement is only for purposes of adjusting and maintaining the pressure at a substantially constant level; not for changing the pressure in any given time cycle or increment.
The fourth type of wave form is peristaltic and is shown in FIG. 1D. The most distal cell is inflated to a tourniquet pressure, followed sequentially by inflating the second most distal cell to the tourniquet pressure. The most distal cell is deflated once the second most distal cell has reached the tourniquet pressure. The process continues in this fashion for each neighboring pair of cells until the most proximal cell is reached. Thus the cells are peristaltically filled which helps ensure proximally directed flow and reduce backflow. Again the inflation of each cell is controlled as a function of time, not pressure. A disadvantage of this method is that since the cells are inflated to tourniquet pressure, they cannot remain inflated for very long. The relatively short compression time reduces the efficiency of the method.